Which Philistine champion from Gath did David defeat?

Answer: Goliath

Goliath, the Philistine champion from Gath, fell facedown when David's stone sank into his forehead.

“Then a champion named Goliath, who was from Gath, came out from the Philistine camp. He was six cubits and a span in height,”— 1 Samuel 17:4 (BSB)